This invention relates to electrodes for use in alkaline storage batteries and more particularly to electrodes which contain nickel active material and which additionally contain graphite as a conductive diluent to impart electronic conductivity to the active material.
Nickel electrodes of this type usually employ Ni(OH).sub.2 as the active material and are commonly employed in alkaline batteries containing Cd, Zn, H.sub.2, etc. as the counter electrode or anode. The conductive graphite diluent in these electrodes takes the place of the more conventional sintered nickel plaque structures and results in a lighter weight and less costly battery system.
One of the major drawbacks to the wide spread use of this electrode is the occurrence of graphite oxidation which appears during long-term cycling, particularly, during charge, since oxygen is evolved on overcharge. Graphite oxidation results in loss of conductivity of the nickel electrode which manifests itself as electrode swelling, increased polarization, lower utilization of the Ni(OH).sub.2 active material and reduced cycle life. Oxidation of graphite in alkaline electrolyte also results in increased K.sub.2 CO.sub.3 concentration, since CO.sub.2 which is the product of oxidation reaction further reacts with KOH to form K.sub.2 CO.sub.3. In Ni-Cd alkaline cells, high carbonate concentrations increase the solubility of Cd(OH).sub.2 resulting in cadmium shorting during cycling which, in turn, reduces cycle life.
Attempts at eliminating or greatly reducing graphite oxidation have included use of coarser graphite particles, chemical or heat treatment of the graphite and combinations of these procedures. All have resulted in little or no improvement in the rate of graphite corrosion.
Still other techniques include the use of mixtures of coarse and fine graphite powders containing metallic powder (e.g. Ni, Mn, Ag, etc.) additions. The addition of metallic elements with low oxygen overvoltages tends to improve somewhat the graphite stability, but these elements are also oxidized with time and decrease the coulombic efficiency of the nickel electrode.
It is, therefore, an object of the present invention to provide a nickel electrode containing a graphite diluent which exhibits longer life.
It is a further object of the present invention to provide a nickel electrode containing a graphite diluent which exhibits decreased oxidation of the diluent.
It is yet a further object of the present invention to provide a nickel electrode containing a graphite diluent in an amount heretofore not achievable.